The present invention relates to materials that encourage elimination, by adsorption, of oxides of nitrogen (NO and NO2, usually termed NOx) present in a gas mixture which may be super-stoichiometric in oxidising compounds, and in particular in oxygen, said materials not being poisoned by the sulphur-containing products present in those gases. The invention is applicable to eliminating oxides of nitrogen (NOx) present in the exhaust gases from automotive vehicles, in particular from vehicles functioning with diesel fuel.
The high toxicity of oxides of nitrogen and their role in the formation of acid rain and tropospheric ozone have led to the instigation of strict regulations limiting the discharge of such compounds. In order to satisfy those regulations, it is generally necessary to eliminate at least a portion of such oxides present in exhaust gases from automotive or stationary engines and from turbines.
The elimination of oxides of nitrogen by thermal decomposition or, as is preferable, by catalytic decomposition can be envisaged, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. Only catalytic reduction of oxides of nitrogen to nitrogen is possible using the reducing agents which are present, albeit in small quantities, in the exhaust gases (CO, H2, unburned hydrocarbons or where combustion in the engine has been imperfect), and also by injecting a complement to those reducing compounds upstream of the catalyst. Such reducing agents are hydrocarbons, alcohols, ethers or other oxygen-containing compounds; they can also be a liquid or gaseous fuel (under pressure, CNG, or liquefied, LPG) feeding the engine or turbine.
European patent EP-A1-0 540 280 describes an apparatus for reducing emissions of oxides of nitrogen in the exhaust gases from internal combustion engines, which comprises a material for adsorbing and desorbing oxides of nitrogen. In that process, the oxides of nitrogen are stored in the form of nitrates when the engine is burning lean, i.e., depleted in hydrocarbons. However, the storage capacity of a trap operating using that principle is generally deteriorated by adsorption of sulphur-containing products contained in the exhaust gas, which form sulphates that are more stable than the nitrates, poisoning the trap.
Further, following NOx trapping, a step for desorbing the oxides of nitrogen must be carried out followed by their reduction. Devices for catalysed oxidation treatment of carbon monoxide CO and hydrocarbons HC contained in the exhaust gases are known which, for example, use catalysts for reducing oxides of nitrogen, known as DeNOx catalysts, which are active for reducing NOx in temperature ranges in the range 200xc2x0 C. to 350xc2x0 C. and which comprise, for example, precious metals on oxide supports such as platinum or palladium deposited on an alumina, titanium oxide or zirconium support, or by perovskites, or in temperature ranges in the range 350xc2x0 C. to 600xc2x0 C. comprising, for example, hydrothennally stable zeolites (for example Cu-ZSM5). A device for treating exhaust gases from a compression ignition engine comprising a catalyst and an oxides of nitrogen adsorbent placed in the exhaust collector has been described, for example, in patents EP-A1-0 540 280 and EP-A1-0 718 478.
Thus, a material behaving as a trap for oxides of nitrogen has to be capable of adsorbing the oxides of nitrogen at low temperatures up to the temperature necessary for the NOx reduction catalyst to function, the trap then allowing the oxides of nitrogen coming into contact with the DeNOx catalyst to desorb at a temperature sufficient to trigger the NOx reduction reaction.
Oxides with a crystallographic structure that generates channels with a size sufficient to accommodate oxides of nitrogen have been described (Japanese patents JP09075715A and JP09075718A). The solids used in those patents are mixed oxides with the crystallographic structure of hollandite and specific formulations K3.6Zn1.8Sn6.2O16 and K1.8A1.8Sn6.2O16.
French patent FR-A-2 733 924 describes a material with formula YBa2Cu3O7-x which can integrate the oxides of nitrogen in the mixed oxide composing the material. That patent indicates that the material, after being charged with oxides of nitrogen, is transformed by passing from an orthorhombic structure that is rich in oxygen to a tetragonal structure that is depleted in oxygen when the oxygen content of the gas reduces, and that phase transition causes desorption of oxides of nitrogen. According to that process, it is possible to influence adsorption and desorption of the oxides of nitrogen by varying the amounts of oxygen in the exhaust gases. It has recently been demonstrated (K-Y Lee, K. Watanabe, M. Misono, Applied Catalysis B 13, 241 (1997)) that the adsorption of NOx in the presence of oxygen on the material YBa2Cu3O7-x leads to the formation of barium nitrate species (Ba(NO3)2). That same study also showed that that material suffers a dramatic loss of its oxides of nitrogen adsorption properties in the presence of carbon dioxide by forming barium carbonates. Since barium sulphate species are more stable than the nitrate species, it is feared that a compound of the YBa2Cu3O7-x type is also poisoned in the presence of sulphur dioxide by forming sulphate species on the oxides of nitrogen adsorption sites.
The materials of the present patent can be found in the natural state or they can be synthesised in the laboratory. EP-A-0 623 556 and EP-A-0 710 622, for example, describe methods for producing certain of such solids. Further, among such solids, materials based on manganese and with channels with sufficient diameters for inserting NOx have been described in the Gmelin Handbook (Gmelin Handbook of Inorganic and Organometallic Chemistry, Mn nxc2x0 56, A5b1, Springer-Verlag, 1996).
The invention concerns materials for eliminating oxides of nitrogen NO and NO2 (NOx) in particular those present in exhaust gases, for example internal combustion engines of automotive vehicles operating in a medium which is super-stoichiometric in oxidising agents, said materials being capable of adsorbing NOx and which can desorb NOx by raising the temperature or by treatment with a mixture that is rich in reducing agents. The materials are mixed oxides the framework of which is constituted by metal cations M each surrounded by 6 oxygen atoms and wherein the octahedra (MO6) thus formed are connected together by edges and peaks generating a structure that produces channels in at least one direction in space. Said materials are of the OMS structure type with a hollandite (OMS 2*2), romanechite (OMS 2*3) or todorokite (OMS 3*3) type structure.
The material of the invention can trap oxides of nitrogen at low temperatures and desorb them at the temperature at which a DeNOx catalyst is capable of reducing them. These materials are insensitive to the oxides of sulphur and carbon contained in the exhaust gases, which prevents the materials from being poisoned. The materials adsorb oxides of nitrogen over a wide temperature range while desorption is carried out in a very narrow temperature range, which means that thermal regeneration is easy to control. During desorption, the oxides of nitrogen that have been adsorbed are released in bursts with a high NOx concentration, which is beneficial to the reaction kinetics for reduction of the desorbed oxides of nitrogen. The kinetics of the reduction of NOx by hydrocarbons are positive with respect to the oxides of nitrogen species. Said material does not have a basic oxide phase that substantially stabilises the oxides of nitrogen and oxides of sulphur into the nitrate and sulphate forms respectively. The SOx that can be inserted with the NOx into the structure of the material of the invention are desorbed in a temperature range that is similar to that of the NOx. Preventing the formation of stable sulphates ensures that poisoning of the adsorbing material is minimal, meaning that the regeneration frequency and the regeneration temperature are lower, and thus the service life of the NOx trap is longer, and there an energy gain.
The material of the invention can also allow chemical desorption by varying the chemical composition of the gas containing the oxides of nitrogen. In a particular implementation of the invention, combining the materials claimed by the Applicant with a metal from group VIII or a suitable metal such as iron, zinc or aluminium eliminates adsorbed NOx by reduction during passage of a rich mixture.
The present invention concerns materials for adsorbing and desorbing oxides of nitrogen the structure of which is composed of octahedra (MO6), M being selected from elements from groups IIIB to IIIA in the periodic table. Preferably, this element (M) has an oxidation number of close to 4. Said material has a characteristic structure that generates channels into which the NOx can insert at low temperatures and leave at a higher temperature. The sides of these channels are formed by linking the octahedra (MO6), which connect together by the edges, these sides connecting themselves together via the peaks of the octahedra. Thus, the width of the channels can vary depending on the mode of preparation, depending on whether the sides are composed of 2 and/or 3 octahedra (MO6). This type of material is known by its acronym OMS, Octahedral Molecular Sieves. In accordance with the invention, the materials are selected so that they have a structure that generates channels either with a square cross section composed of two octahedra by two octahedra (OMS 2*2) or three octahedra by three octahedra (OMS 3*3), or with a rectangular cross section composed of two octahedra by three octahedra (OMS 2*3). The materials are selected from materials with a hollandite (OMS 2*2), romanechite (OMS 2*3) or todorokite (OMS 3*3) type structure.
More precisely, the adsorbing phase of materials with type OMS 2*2, OMS 2*3 and OMS 3*3 of the invention have a three-dimensional structure that generates channels in at least one direction in space. Said phase is composed of octahedra (MO6) and comprises:
at least one element (M) selected from the group formed by elements from groups IIIB, IVB, VB, VIB, VIIB, VIII, IB, IIB, IIIA of the periodic table and germanium, each element M being coordinated with 6 oxygen atoms, and located at the centre of the oxygen octahedra;
at least one element (B) selected from the group formed by the alkali elements IA, the alkaline-earth elements IIA, the rare earths IIIB, transition metals or elements from groups IIIA and IVA, element B generally being located in channels in the oxide structure.
Particularly preferred materials of the invention of type OMS2*2, OMS2*3 and OMS 3*3 that can be used to eliminate oxides of nitrogen are materials with type OMS2*2.
Preferably, the octahedra contain no tin.
More particularly, elements M are selected from scandium, yttrium, lanthanum, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, zinc, cadmium, aluminium, gallium, indium, and mixtures of at least two of these elements.
The average charge (oxidation number) carried by the cation or cations M from groups IIIB to IIIA is preferably about +4. At least the major portion of elements (M) is preferably selected from elements in the group constituted by manganese, titanium, aluminium, zinc, copper, zirconium, iron and mixtures of at least two of these elements. Highly preferably, the major portion of elements (M) is selected from manganese, titanium, zirconium and mixtures of at least two of these elements.
Other elements M from groups IIIB to IIIA can be added in minor quantities as dopants. Preferably, the elements from groups IIIB to IIIA added in minor quantities are selected from manganese, molybdenum, aluminium, zinc, copper, nickel, cobalt, iron, chromium, scandium, yttrium, gallium, cadmium and indium and mixtures of at least two of these elements, and more preferably selected from manganese, aluminium, zinc, copper, nickel, cobalt, gallium and mixtures of at least two of these elements. In particular, a manganese-titanium mixture is highly advantageous when the titanium is in the majority and the manganese is in the minority.
An element is in the majority when it satisfies the following formula: (nmaj/xcexa3nM) greater than 1/N, where nM is the number of atoms of element (M) and N is the number of different elements (M) composing the framework, nmaj being the highest number of atoms of element (M).
Elements (B) belong to the group formed by the alkali elements IA, alkaline-earth elements IIA, rare earth elements IIIB, transition metals and elements from groups IIIA and IVA. They are located in the channels of the material. Preferably, metal B is selected from the group formed by potassium, barium, strontium, iron, copper, zinc, aluminium, rubidium, magnesium and calcium and mixtures of at least two of these elements.
In one implementation of the invention, the material also comprises at least one metal (termed C) selected from the group formed by precious metals from the platinum family (group VIII). Elements (C) belong to the group formed by platinum, palladium, rhodium, ruthenium, iridium and osmium; preferably, platinum is selected. This implementation of the invention comprising at least one element (C) selected from noble metals from group VIII can oxidise NO to NO2 and can also reduce the NOx subsequently during adsorption then desorption. This type of material of the invention can thus, surprisingly, carry out the three steps on a single material.
Other combinations of metals can also produce materials that can trap NOx and catalyse the reduction. This is particularly the case with materials with structure type OMS 2*2 in which the centres of the oxygen octahedra occupied by element (M) and/or the sites occupied by element (B) in channels of the oxide structure are occupied by elements with a different nature. In such a case, element (B) is at least partially preferentially selected from the group formed by alkali metals and alkaline-earth metals and element (M) is at least partially preferentially constituted by manganese. The sites that are not occupied by manganese and/or by the alkali metal or alkaline-earth metal can thus enable at least one element to be inserted, preferably selected from the group formed by transition metals and aluminium. Advantageous transition metals are iron, zinc, copper, nickel and cobalt. Iron and zinc are particularly preferred. As examples, materials with structure type OMS2*2 with formulae KZnMn, KFeMn or KAlMn carry out the steps of adsorption, desorption and reduction of NOx, thus avoiding introducing a NOx reduction catalyst upstream of the NOx trap, or broadening the temperature efficiency window for the reduction reaction when used in combination with an element (C).
More particularly, the adsorbing phase of the invention has the following composition by weight, expressed as the percentage by weight with respect to the total mass of this active phase calcined at 1000xc2x0 C. for 4 hours:
30% to 80% of at least one metal M, preferably 40% to 75%, highly preferably 45% to 70%;
0.01% to 30%, preferably 1.5% to 20%, of at least one element (B) selected from the group formed by alkali elements, alkaline-earth elements, rare earth elements, transition metals and elements from groups IIIA and IVA of the periodic table;
optionally, 0.05% to 5% of at least one metal (C) selected from the group formed by the precious metals from group VIII of the periodic table, the complement by weight being formed by the oxygen of the corresponding oxides.
A number of different methods exist for preparing such materials (S. L. Suib, C-L O""Young, xe2x80x9cSynthesis of Porous Materialsxe2x80x9d, M. L. Occelli, H. Kessler, eds, M. Dekker, Inc., p. 215, 1997). They may be synthesised by mixing and grinding solid inorganic precursors of metals M and B, followed by calcining. The materials can also be obtained by heating solutions of precursor salts to reflux, drying and calcining, by precipitating precursor salts by the sol-gel method, or by hydrothermal synthesis which consists of heating an aqueous solution containing the elements constituting the final material under autogenous pressure. The materials obtained from these syntheses can be modified by ion exchange or isomorphous substitution.
Optional metal (C) is introduced using any of the methods known to the skilled person: excess impregnation, dry impregnation, ion exchange, etc.
The material of the invention generally has a specific surface area in the range 1 to 300 m2/g, preferably in the range 2 to 300 m2/g, highly preferably in the range 30 to 250 m2/g. The adsorption kinetics are better when the specific surface area is high, i.e., in the range 30 to 250 m2/g.
The adsorbent phases can be in the form of a powder, beads, pellets or extrudates; they can also be deposited or directly prepared on monolithic supports of ceramic or metal. Advantageously, in order to increase the dispersion of the materials and thus to increase their capacity to adsorb NOx, the materials can be deposited on porous supports with a high specific surface area such as silica or alumina before being formed (extrusion, coating . . . ). These supports are generally selected from the group formed by the following compounds: alumina (alpha, beta, delta, gamma, khi, or theta alumina), silicas (SiO2), silica-aluminas, zeolites, titanium oxide (TiO2), zirconium oxide (ZrO2), magnesium oxide (MgO), divided carbides, for example silicon carbides (SiC), used alone or as a mixture. Mixed oxides or solid solutions comprising at least two of the above oxides can be added.
However, for use in a vehicle, it is usually preferable to use rigid supports (monoliths) with a large open porosity (more than 70%) to limit pressure drops that may cause high gas flow rates, and in particular high exhaust gas space velocities. These pressure drops are deleterious to proper functioning of the engine and contribute to reducing the efficiency of an internal combustion engine (gasoline or diesel). Further, the exhaust system is subjected to vibrations and to substantial mechanical and thermal shocks, so catalysts in the form of beads, pellets or extrudates run the risk of deterioration due to wear or fracturing.
Two techniques are used to prepare the catalysts of the invention on monolithic ceramic or metal supports (or substrates).
The first technique comprises direct deposition on the monolithic support, using a wash coating technique which is known to the skilled person, to coat the adsorbing phase prepared using the operating procedure described, for example, in the reference (S. L. Suib, C-L O""Young, xe2x80x9cSynthesis of Porous Materialsxe2x80x9d, M. L. Occelli, H. Kessler, eds, M. Dekker, Inc., p. 215, 1997). The adsorbent phase can be coated just after the co-precipitation step, hydrothermal synthesis step or heating under reflux step, the final calcining step being carried out on the phase deposited on the monolith, or the monolith can be coated after the material has been prepared in its final state, i.e., after the final calcining step.
The second technique comprises firstly depositing the inorganic oxide on the monolithic support then calcining the monolith between 500xc2x0 C. and 1100xc2x0 C. so that the specific surface area of this oxide is in the range 20 to 150 m2/g, then coating the monolithic substrate covered with the inorganic oxide with the adsorbent phase obtained after the steps described in the reference (S. L. Suib, C-L O""Young, xe2x80x9cSynthesis of Porous Materialsxe2x80x9d, M. L. Occelli, H. Kessler, eds, M. Dekker, Inc., p. 215, 1997).
Monolithic supports that can be used are:
either ceramic, where the principal elements can be alumina, zirconia, cordierite, mullite, silica, alumino-silicates or a combination of several of these compounds;
or a silicon carbide and/or nitride;
or aluminium titanate;
or metal, generally obtained from iron, chromium or aluminium alloys optionally doped with nickel, cobalt, cerium or yttrium.
The structure of the ceramic supports is that of a honeycomb, or they are in the form of a foam or fibres.
Metal supports can be produced by winding corrugated strips or by stacking corrugated sheets to constitute a honeycomb structure with straight or zigzag channels which may or may not communicate with each other. They can also be produced from metal fibres or wires which are interlocked, woven or braided.
With supports of metal comprising aluminium in their composition, it is recommended that they are pre-treated at high temperature (for example between 700xc2x0 C. and 1100xc2x0 C.) to develop a micro-layer of refractory alumina on the surface. This superficial micro-layer, with a porosity and specific surface area which is higher than that of the original metal, encourages adhesion of the active phase and protects the remainder of the support against corrosion.
The quantity of adsorbent phase deposited or prepared directly on the ceramic or metallic support (or substrate) is generally in the range 20 to 300 g per liter of said support, advantageously in the range 50 to 200 g per liter.
The materials of the invention can thus adsorb and desorb oxides of nitrogen present in the gases, in particular exhaust gases, and certain of these materials also have a reducing power as mentioned above.
These materials are characterized in that they are capable of adsorbing NOx at a temperature which is generally in the range 50xc2x0 C. to 450xc2x0 C., preferably in the range 100xc2x0 C. to 350xc2x0 C., more preferably in the range 150xc2x0 C. to 300xc2x0 C. Said oxides of nitrogen can be desorbed at a temperature generally in the range 250xc2x0 C. to 500xc2x0 C., preferably in the range 300xc2x0 C. to 450xc2x0 C. They can also be desorbed by varying the composition of the gas, for example by suddenly increasing the concentration of reducing compounds such as hydrocarbons, hydrogen, carbon monoxide, at temperatures in the range 150xc2x0 C. to 500xc2x0 C., preferably in the range 200xc2x0 C. to 450xc2x0 C., more preferably in the range 300xc2x0 C. to 400xc2x0 C. Thermally or chemically, oxides of nitrogen desorption can be triggered in temperature ranges were conventional NOx reduction catalysts are effective. Further, the thermal desorption of the invention can take place within narrow ranges of temperature generally within a range of 80xc2x0 C. For diesel cars, the temperature of the exhaust gas is generally in the range 150xc2x0 C. to 300xc2x0 C. and rarely exceeds 500xc2x0 C. The materials used in the process of the invention are thus suitable for adsorbing oxides of nitrogen present in the exhaust gases of stationary engines or, particularly, automotive diesel engines or spark ignition (lean burn) engines, but also in the gases from gas turbines operating with gas or liquid fuels. These gases are also characterized by oxides of nitrogen contents of a few tens to a few thousands of parts per million (ppm) and can contain comparable amounts of reducing compounds (CO, H2, hydrocarbons) and sulphur, also large quantities of oxygen (1% to close to 20% by volume) and steam. The material of the invention can be used with HSVs (hourly space velocity, corresponding to the ratio of the volume of the monolith to the gas flow rate) for the exhaust gas generally in the range 500 to 150000 hxe2x88x921, for example in the range 5000 to 100000 hxe2x88x921.
It may be particularly advantageous to provide a mixture of at least two materials of the invention to combine the advantages of each with a restricted range over a wider temperature range. For the mixture of at least two materials of the invention, it is possible either to mix distinct powders of two materials and coat a monolith using a suspension of a mixture of the powder or to join two monoliths together, each monolith being coated with a suspension containing a single material.
The invention also concerns the use of materials for adsorbing and desorbing oxides of nitrogen in a process for eliminating oxides of nitrogen, more particularly in a medium that is super-stoichiometric in oxidising agents. Thus, the material of the invention can be used in a process comprising:
a step for adsorbing at least a portion of said oxides of nitrogen onto an adsorption material as defined in the present invention;
a step for desorbing the oxides of nitrogen carried out by increasing the temperature or by varying the composition of the exhaust gases;
a step for selective reduction of at least a portion of the oxides of nitrogen to molecular nitrogen by reducing agents in the presence of at least one catalyst for reducing oxides of nitrogen.
Thus, the process for eliminating oxides of nitrogen comprises, during the step for reducing the oxides of nitrogen, the use of a catalyst which is active and selective using reducing agents to reduce oxides of nitrogen to molecular nitrogen in a medium which is super-stoichiometric in oxidising agents. Catalysts for reducing oxides of nitrogen to nitrogen or nitrous oxide generally comprise at least one inorganic refractory oxide and can comprise at least one zeolite selected, for example, from MFI, NU-86, NU-87 and EU-1 zeolites and generally at least one element selected from elements from transition metal groups VIB, VIIB, VIII and IB. These catalysts can optionally contain at least one element selected from noble metals from group VIII, for example platinum, rhodium, ruthenium, iridium, palladium and optionally at least one element selected from elements from groups IIA, the alkaline-earths and IIIB, the rare earths. Examples of catalysts for reducing oxides of nitrogen include the following combinations: Cu-ZSM5, Cu-MFI, Fe-MFI, Fe-ZSM5, Ce-MFI, Ce-ZSM5, Pt-MFI, Pt-ZSM5.
The refractory inorganic oxide is selected from supports of the type Al2O3, SiO2, Zro2 and TiO2, preferably alumina.
The reducing agents are selected from CO, H2, hydrocarbons, present in the fuel or added in the form of fresh products.
In the case where the material for adsorbing oxides of nitrogen of the present invention contains at least one element (C) selected from noble metals from group VIII of the periodic table or contains, as at least element (M) and/or element (B), an element selected from transition metals and aluminium in addition to another element (B) preferably selected from alkali metals and alkaline-earth metals and a further element (M) generally constituted by manganese, the process for eliminating oxides of nitrogen comprises:
a step for adsorbing at least a portion of said oxides of nitrogen on the material as defined in the present invention;
a step for desorbing the oxides of nitrogen;
a step for selective reduction of at least a portion of the oxides of nitrogen to molecular nitrogen in the presence of reducing compounds on the material as defined in the present invention.
Thus, reducing oxides of nitrogen to nitrogen or nitrous oxide can take place directly on the adsorption material of the invention, which permits both trapping of the oxides of nitrogen, desorption of said oxides of nitrogen and reduction thereof.